Scanner element for coulter particle apparatus

ABSTRACT

The scanner element or aperture tube of a Coulter particle apparatus has a wafer in which the aperture is formed, the wafer being made of a material which has electrical insulating properties but high heat conductivity. The tube is of glass and the wafer is set into the side wall of the tube. The result is a scanner element of increased sensitivity. In one form of the invention, the surfaces of the tube, at least in the vicinity of the wafer, are covered by a coating of highly conductive material coming close to but not engaging within the aperture of the wafer. The coating inside and outside of the tube will comprise electrodes in the bodies of liquid respectively disposed on opposite sides of the tube wall.

Nov. 6, 1973 SCANNER ELEMENT FOR COULTER PARTICLE APPARATUS [75]Inventor: Walter R. Hogg, Miami Lakes, Fla.

[73] Assignee: Coulter Electronics, Inc., Hialeah,

Fla.

[22] Filed: Apr. 5, 1971 [21] App]. No.: 131,361

[52] US. Cl. 324/71 CP [51] Int. Cl. G01n 27/00 [58] Field of Search324/71, 71 CP, 30; 23/252; 65/36, 42; 138/103; 204/274, 106

[56] References Cited UNITED STATES PATENTS 2,861,932 11/1958 Pohl204/274 3,345,561 10/1967 3,628,140 12/1971 2,985,830 5/1961 3,457,5017/1969 3,266,526 8/1966 3,238,452 3/1966 Schmitt 324/71 CP OTHERPUBLICATIONS Handbook of Chem. & Physics, 1966, p. 15-4, 47 Ed., CRC.

Primary ExaminerRudolph V. Rolinec Assistant Examiner-Emest F. KarlsenAttorney-Silverman & Cass 57 ABSTRACT The scanner element or aperturetube of a Coulter particle apparatus has a wafer in which the apertureis formed, the wafer being made of a material which has electricalinsulating properties but high heat conductivity. The tube is of glassand the wafer is set into the side wall of the tube. The result is ascanner element of increased sensitivity. In one form of the invention,the surfaces of the tube, at least in the vicinity of the wafer, arecovered by a coating of highly conductive material coming close to butnot engaging within the aperture of the wafer. The coating inside andoutside of the tube will comprise electrodes in the bodies of liquidrespectively disposed on opposite sides of the tube wall.

17 Claims, 6 Drawing Figures SCANNER ELEMENT FOR COULTER PARTICLEAPPARATUS CROSS-REFERENCE TO RELATED APPLICATION One aspect of theinvention herein comprises an improvement upon the structures of acopending application owned by the assignee of this application andentitled Electronic Particle Analyzing Apparatus with Improved ApertureTube," Ser. No. 128,332, filed Mar. 26, 1971.

BACKGROUND OF THE INVENTION ture while at the same time an electriccurrent is flowing through the aperture. Each time that a particlepasses through the aperture it changes to effective impedance of thebody of liquid which is subjected to the influence of the field in theaperture and thereby produces a signal which can be detected for makingstudies of population, concentration, size, etc. of the particulatesystem in suspension.

. The basic Coulter apparatus is disclosed in U.S. Pat. No. 2,656,508.The apparatus includes a body of sample. suspension retained in vesselof insulating material and a so-called aperture tube immersed in thevessel. The aperture tubehas a small wafer set into its wall close tothe bottom of the tube, which is usually made of glass, the wafercommonly being made of corundum, and the interior of the aperture tubeis filled with liquid also. The usual arrangement includes a closedliquid system of which the interior of the aperture tube comprises apart. Such a system is disclosed in U.S. Pat. No.

2,869,078 and provides for means to cause the flow of the suspensionfrom the outer vessel through the aperture in theaperture tube while anelectric current also flows through the aperture. There is an electronicdetector which is coupled to the respective bodies of liquid on theinterior and exterior of the aperture tube by means of metal electrodesimmersed in the respective bodies of liquid. The source of electriccurrent is also connected to these electrodes.

The aperture which is formed in the aperture tube is a minute hole in acorundum wafer thatis set into the wall of the tube. The aperture tubebecomes a scanner element since it scans the liquid flowing through itsaperture and produces a measurable signal each time that a particlepasses through. The construction of the aperture tube and one method ofsetting the wafer into the side wall of the tube are disclosed in U.S.Pat. Nos. 2,985,830 and 3,l22,43l.

An understanding of the invention herein will be more readily obtainedand its advantages fully appreciated from a discussion of the nature ofthe scanner element with respect to its requirements.

The presence of an electric current passing through the apertureproduces a concentrated electric field in a zone which includes theentire aperture and slight bulges at its opposite ends. The currentdensity outside trolyte of the two liquid bodies in which the electrodesare immersed. This zone, which may be called a sensing zone, is thevolume of electrolyte whose impedance is changed by the presence of aparticle. If the energy in the sensing zone is provided by a lowfrequency source of electrical current and the effective electricalimpedance of the particles is several orders of magnitude removed fromthat of the electrolyte (which is practically the case most of thetime), then the change in impedance of the effective volume of thesensing zone by the introduction of the particle thereinto will producea signal which can be detected which. is substantially independent ofthe shape and orientation of the particle. The principle described abovesignifies that the signal is proportional to the size or volume of theparticle. The linearity of response versus: particle size is best underconditions that the particles are small with -respect to the aperture,for example, having effective diameters less than ten percent of theaperture diameter. Above that size, departure from linearity becomesmore apparent but not to the extent that corrections cannot be made inresults.

The types of particles which have been analyzed by means of the Coulterapparatus cover a very wide gamut and include biological and industrialparticles, as well-known in this art. In any given study, one willchoose an aperture diameter to provide fairly linear output for thelargest particles which are expected to be involved, but this choice isa compromise with the desire to detect the smallest useful particles aswell. In the latter case, the aperture cannot be too large because itssensitivity decreases with increase in size. This should be obvioussince the current density decreases for larger apertures. The length ofthe aperture is generally made about to 100 percent of its diameter,primarily to give the central region of the electric field within theaperture an opportunity to become fairly uniform. It has been mentionedabove that the field bulges at the ends of the aperture giving effectswhich decrease the sharpness of the signal and its uniformity. Theaverage length of an aperture is about percent of its diameter.

Longer apertures provide problems which offset their advantages. Theadvantages are a small increase in field uniformity in the center of theaperture and a decrease in the required bandwidth of-the amplifiers usedin the detector of the Coulter apparatus. The disadvantages are thegreater likelihood that coincidence of more than one particle in theaperture will occur; an increased likelihood of debris pluggng theaperture with greater difficulty of dislodging the debris; and anincrease in the resistance of the longer path. This latter disadvantageis of especially greater importance since it relates to the inventionherein.

Increased resistance in the aperture will generatemore socalled Johnsonnoise than a lower resistance of a shorter path thus cancelling the gainto be achieved due to decreased bandwidth of the ampli'fieroTheincreased resistance also is part of the problem of heating of theelectrolyte as it passes through the aperture. The current density inthe aperture is very high and the electrolyte remains under thisinfluence for a longer time than in the case of shorter apertures.Heating of the electrolyte will cause it to produce noise components ofa random nature above "the normal Johnson noise limiting the size ofparticles which can be detected to those which are large enough toproduce signals greater than the noise. Additionally, should thetemperature of the electrolyte rise above the boiling 'point, smallbubbles will be generated in the aperture and these appear as particlesto the detector.

It should be recognized that while the Johnson noise of the contents ofthe aperture is relatively constant for the rather narrow range oftemperatures normally encountered, being proportional to the square rootof the temperature on the Kelvin scale, the electrical signal generatedby the passage of a particle is proportional to the intensity of theaperture current. If these were the only considerations, it would bepossible to detect any particle so long as it exceeded by several ordersof magnitude the ionic dimensions of the electrolyte used, and displacedenough ions to cause a discernible change over and above the randomfluctuation in the number of ions in'thesensing zone. The heating of theelectrolyte, however, eventually limits the usefulness of increasedaperture current as will be seen hereinafter.

It has been found that the type of aperture which is best used inCoulter apparatus is one which has a sharpedged inlet. Obviously, in themanufacture of the wafers which are set into the glass tubes, one makesboth ends of the wafers sharp-edged because of the practical problems ofidentifying which is the sharp-edged entrance in the case only one endwere sharp-edged. These types of apertures are easy to clear in theevent that debris becomes lodged in them, which is just the opposite ofwafers that have funnel-shaped entrances. The sharp-edged wafers areeasier to manufacture and inspect.

The effect of such sharp-edged inlets upon the flow of liquid throughthe aperture is to produce a pattern of flow that is known as venacontracta. The flow pattern commences to constrict at the entrance andgrows progressively smaller downstream of the entrance, leaving a spacebetween the vena contracta and the wall of the aperture in which theelectrolyte has no definite velocity, certainly not the average velocityof the stream passing through the axis of the aperture. The electrolytein this region has eddy currents in it, tha part being swept out byreason of proximity to the vena contracta being replaced by electrolytewhich enters the region from the downstream end of the aperture next toits walls. This effectively stagnant region has no organized flowpattern and has substantially less motion than the main flow of liquid.

It will be recalled that simultaneously with fluid flow, there is anelectric current flowing in the aperture, generating heat in theelectrolyte. The temperature of any volume increment of electrolyterises in accordance with its stay in the region of high current density.It follows that the central laminar flow of the vena contracta willproduce thecoolest electrolyte but that the electrolyte in thequasi-stagnant region described above will have increments ofelectrolyte of higher temperatures and of differing temperaturesdepending upon how long they remain in the aperture.

The electrical conductivity of an electrolyte varies with itstemperature quite rapidly. For instance, a 0.1 normal solution ofpotassium chloride at 3 lC has double the conductivity that it has at C.Thus, an appreciable proportion ofthe contents of the aperture has anunpredictable conductivity when high aperture currents are used, a factwhich causes random modulation of the aperture resistance which-is inturn interpreted by theapparatus as noise. In additionto the-simple.

modulation of the aperture resistance due to changes in conductivity,the temperature rises in various locations within theaperture may permitthe release of occluded gases in the form of microscopic bubbles, whichdisplace electrolyte and hence are interpreted by the apparatus asparticles. Volatile electrolyte may boil, as mentioned above, and thesebubbles also produce signals which look like particle pulses.Accordingly, there is an optimum value of aperture current beyond whichthe phenomena described are intolerable.

From the above discussion, it will follow that the signal-to-noise ratioof the Coulter apparatus improves linearly with aperture current forsmall aperture currents since the noise is constant whereas the signaldeveloped is proportional to aperture current. Sensitivity alsoincreases. The point is reached, however, at which in addition to theJohnson noise, noise due to the heating effects described above,increase at the same rate as the signal, beyond which point no furtherimprovement is gained in the signal-to-noise ratio. As a matter of fact,noise increases more rapidly than the signal after this latter mentionedpoint is reached so that the signal-to-noise ratio is instead worsened.

The invention herein provides a structure for a scanner element whichresults in cooling the electrolyte located in the region onquasi-stagnation, thereby lessening the degree of modulation ofresistance for a given aperture current. Absent the heating noise,sensitivity and signal-tonoise ratio are substantially improved with theresult that a given aperture is capable of distinguishing between verymuch smaller particles than have heretofore been detected by the normalCoulter apparatus. It can be seen that an entire new field of particletechnology can be opened by theinvention.

Just, for example, up to the present time by careful control of aperturecurrent and the use of relatively small apertures, it has been feasibleto study particles of the order of one micron in size. Below thisapproximate range, noise prevented distinguishing between smallparticles. The invention enables studies to be made of particles whichare appreciably smaller than was heretofore possible.

SUMMARY OF THE INVENTION The scanner element of the invention ischaracterized by the provision of an aperture wafer that is formed of ahard material that is insulative with respect to the electrical currentflowing in the aperture but which has high thermal conductivity.Accordingly, the wall of the aperture conducts heat away from theaperture thereby cooling the electrolyte in the quasistagnant regionthat is in closest contact with the wall. For very small apertures, thewafer itself and the electrolyte in contact with it may function as aheat sink. In the case of larger apertures, metallic means may be usedin contact with the wafer to serve as a heat conduit. The material ofthe wafer, in addition to the above qualities, must be capable ofsecurement to the wall of anaperture tube by some suitable means. Theexample described herein is beryllium oxide.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic viewillustrating lines of flow of a liquidthrough an aperture of any type;

FIG. 2 is a fragmentary sectional view through a "scanner-element of theinvention illustrating one ing-"the aperture wafer to the side wall;

same:

FIG. 3 is a fragmentary front elevational view of the FIG. 4 is afragmentary sectional view through a scanner element showing anothermethod of securing the aperture wafer to the side wall thereof;

FIG. 5 is a fragmentary sectional view through a modified form ofscanner element; and

FIG. 6 is a diagrammatic view showing a conventional Coulter apparatussetup using a scanner element of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT conventional wafer of theCoulter apparatus such as, for example, the wafer 10 has at least itsentrance edge 12 opening to the aperture 14 sharp. It is presumed, inthis case, that there is liquid on opposite sides of the wafer but thisis not diagrammed in order to render the illustration clear. This isalso true of the other illustrations herein.

The flow of. liquid through such an aperture 14 is characterized by aflow field illustrated, the flow of FIG. I being assumed to be in thedirection of the arrows. Jet contraction and flow curvatures areproduced by the radial approach of fluid to the aperture 14, this beingillustrated in region 16 after which the streamlines become essentiallystraight and parallel at a section termed the vena contracta a shortdistance downstream from the entrance 12. In the illustration of FIG. 1,the vena contracta is shown at approximately the region 18 but it maywell occur within the bore of the aperture 14 shortly after the liquidhas passed the entrance 12. a

. In any event, it will be seen that the principal flow of liquidthrough the aperture 14 is radially inward of the aperture wall 20thereby producing a region of turbulence or quasi-stagnation at 22.Liquid is detached from the downstream end of the main flow to replaceelectrolyte drawn into the flow as indicated by the small arrows 24. Itis in this region 22 that heating noise is produced as the aperturecurrent is increased due to thefact that the time that increments ofelectrolyte remain in the aperture 14 in these regions 22 is muchgreater and more variable than the time that an increment in theprincipal flow remains in the aperture.

FIGS. 2 and 3 illustrate a scanner element 26 which is constructed inaccordance with the invention. As in the case of the usual scannerelement or aperture tube of a Coulter apparatus, there is a tube 28 madeof a principally transparent material such as glass. The glass wall ofthe tube is designated 30 and any of the known techniques may be used toprepare the wall 30 for receiving the aperture wafer. The wafer 32 inFIGS. 2 and 3 has been fused into the outer surface of the wall 30immediately over a large orifice 34 that is formed in the wall 30. Thewafer 32 has a central aperture 36 not much different in configurationthan the aperture 14 of FIG. 1 and probably formed in the wafer 32 bysimilar technique. The wafer 32 is made of a meterial which iselectrically insulative and yet heat-conductive such as, for example,beryllium oxide or diamond. Of these, beryllium oxide is preferred,since it is relatively inexpensive, about the same hardness as corundum,its thermal conductivity is greater than that of many metals and it iseasy to handle and fuse to glasses having the same or slightly greatercoefficients of thermal expansion, such as some soda-lime glasses. It isdesirable for the glass to have slightly higher rates of expansion sothat the beryllium oxide is in compression when the assembly cools. Incase of apertures substantially. less than microns in diameter, thewafer 32 will serve as a heat sink together with the electrolyte whichcontacts the same and draws heat from the region of quasi-stagnancy suchas 22 illustrated in FIG. 1. This cooling effect decreases heat noiseand enables higher aperture currents to be used with resulting increasedsensitivity to smaller particles.

FIG. 4 illustrates a similar wafer 32 but in this case the wafer issecured to the outer surface of the wall 30 by means of an adhesive 38such as, for example, an epoxy type. Some epoxy type adhesives have veryhigh thermal conductivity and good electrically insulative properties.

FIG. 5 illustrates a scanner element 40 having an orifree 42 in its wall44 and having an aperture wafer 46 fused into the wall over the orifice.The aperture wafer 46 has a central aperture 48 and the material fromwhich the wafer 46 is made has good electrically insulative propertiesand high thermal conductivity.

In this case, it is assumed that the aperture 48 is of the order of 100microns or greater. A metallic coating is shown at 50 on the exteriorand 52 on the interior of the wall 44 with portions 54 and 56 overlyingthe surface of the wafer 46 and being in intimate contact therewith.Such coatings could also be in the form of wires, strips or bars. Inthis case, the heat generated in the aperture 48 is conducted by thewafer 46 and the metallic members 54 and 56 to the electrolyte which isdisposed on both sides of the wall 44. These metallic heat-conductingmembers 50 and 52 need not be connected to the electrodes leading; tothe detector of Coulter particle device. Under such circumstances, themetallic coatings or connections serve as conduits of heat, drawing samefrom the wafer to the electrolyte bodies. They may also serve as suchelectrodes and be connected as shown by leads 58 to the Coulterparticledevice. The full coating is especially useful in case of high frequencyaperture current as disclosed in said copending application.

In addition to the structures illustrated herein, the copendingapplication illustrates a form of scanner element or aperture tube inwhich the walls are made of synthetic resin enclosed within metallictubes that serve as the inner and outer electrodes. In such case, thewafer used is required to be set into the wall of the aperturetube in asuitable cavity provided such as, for example, at the bottom end. Thismay also be done with an aperture wafer of the material disclosedherein.

As will be obvious from the above discussion, the invention may beembodied in scanner elements of many different constructions. Inaddition to the type of Coulter apparatus in which intermittent flow isachieved by means of such systems as disclosed in US. Pat. No.2,869,078, the wafers of the invention may be utilized in continuousflow structures and, in fact, practically anywhere that a Coulterscanner element is used.

FIG. 6 shows a conventional arrangement using an aperture tube such as26 with an aperture wafer 32 in the side wall thereof near the lowerend, set into a vessel 60 having a suspension 62 of particles therein.The interior of the aperture tube 26 has a second body of fluid 64therein and is connected into a system of the type disclosed in US. Pat.No. 2,869,078. Electrodes 66 and 68 connect to the Coulter particleanalyzing device 70. The suspension 62 flows through the aperture of thewafer 32 to the body of fluid 64 and passage of particles is detected bythe apparatus 70.

It is preferred that the Wall of the aperture tube 26 by transparent sothat the image of the aperture 36 may be optically projected onto somesurface for viewing during use.

What it is desired to secure by Letters Patent of the United States is:

1. In a particle studying apparatus the improvement comprising a scannerelement including a wall of electrically insulative material having anorifice therein and a wafer being of a flat disc configuration andhaving an aperture therein secured to said wall over said orificewhereby liquid flow from one side of the wall to the other will passthrough said aperture, while an electric current also flows through theaperture, a signal being produced each time a particle passes throughthe aperture, said wafer being of a material of electrically insulativeproperty and thermal conductivity of at least 50 Btu/hr/sq ft/F/ft, suchas to reduce excessive noise in the aperture, improve thesignal-to-noise ratio, and increase sensitivity of the scanner element.

2. The scanner element as claimed in claim 1 which the wafer is fusedlysecured to said wall.

3. The scanner element as claimed in claim 1 which the wafer is adheredto said wall.

4. The scanner element as claimed in claim 1 which the wafer is formedof an oxide of beryllium.

5. The scanner element as claimed in claim 2 which the wafer is formedof an oxide of beryllium.

6. The scanner element as claimed in claim 3 which the wafer is formedof an oxide of beryllium.

7. The scanner element as claimed in claim 4 which the wall is formed ofglass.

8. The scanner element as claimed in claim 7 which the glass is of thesoda-lime variety.

9. The scanner element as claimed in claim 1 in which the wafer hasmetallic means engaged therewith to serve as heat conduit means, saidmetallic means comprising separate members engaged against oppositefaces of said wafer.

10. The scanner element as claimed in claim 9 in which said separatemembers surround said aperture at opposite ends thereof and come inclose proximity thereto.

11. The scanner element as claimed in claim 9 in which said separatemembers comprise electrodes adopted to be connected to a Coulterparticle device.

12. in particle studying apparatus which includes a vessel having anaperture therein through which fluids carrying suspensions of particlesare adapted to pass, the passage of a particle through the apertureresulting in a change of the impedance of the fluid in the aperture, andin which electrical means are provided to detect the passage ofparticles in terms of the change in impedance, the improvementcomprising, a scanner element which comprises a wall of insulatingmaterial having an orifice therein, a substantially inert wafer having aflat disc configuration and being of electrically insulative materialand having thermal conductivity of at least 50 Btu/hr/sq ft/F/ft andhaving an aperture therein secured to said wall in face-to-faceengagement and with the aperture and orifice aligned, wherein excessivenoise in the aperture is reduced, the signal-tonoise ratio is improved,and the sensitivity of the scanner elmement is increased.

13. The structure as claimed in claim 12 in which the aperture is ofright cylindrical configuration and having a sharp entrance edge.

14. The structure as claimed in claim 12 in which the wafer is formed ofan oxide of beryllium.

15. The structure as claimed in claim 13 in which the wafer is formed ofberyllium oxide.

16. An aperture tube for use with a Coulter particle apparatuscomprising an elongate glass tube having an orifice in its wall adjacentthe lower end thereof, an electrically insulative wafer being of a flatdisc configuration and having thermal conductivity of at least 50Btu/hr/sq ft/F/ft engaged to said wall and blocking said orifice andthere being a through aperture in said wafer aligned with said orifice,wherein excessive noise in the aperture is reduced, the signal-to-noiseratio is improved, and the sensitivity of the aperture tube isincreased.

17. The aperture tube as claimed in claim 15 in which the wafer isformed of an oxide of beryllium.

1. In a particle studying apparatus the improvement comprising a scannerelement including a wall of electrically insulative material having anorifice therein and a wafer being of a flat disc configuration andhaving an aperture therein secured to said wall over said orificewhereby liquid flow from one side of the wall to the other will passthrough said aperture, while an electric current also flows through theaperture, a signal being produced each time a particle passes throughthe aperture, said wafer being of a material of electrically insulativeproperty and thermal conductivity of at least 50 Btu/hr/sq ft/*F/ft,such as to reduce excessive noise in the aperture, improve thesignal-tonoise ratio, and increase sensitivity of the scanner Element.2. The scanner element as claimed in claim 1 in which the wafer isfusedly secured to said wall.
 3. The scanner element as claimed in claim1 in which the wafer is adhered to said wall.
 4. The scanner element asclaimed in claim 1 in which the wafer is formed of an oxide ofberyllium.
 5. The scanner element as claimed in claim 2 in which thewafer is formed of an oxide of beryllium.
 6. The scanner element asclaimed in claim 3 in which the wafer is formed of an oxide ofberyllium.
 7. The scanner element as claimed in claim 4 in which thewall is formed of glass.
 8. The scanner element as claimed in claim 7 inwhich the glass is of the soda-lime variety.
 9. The scanner element asclaimed in claim 1 in which the wafer has metallic means engagedtherewith to serve as heat conduit means, said metallic means comprisingseparate members engaged against opposite faces of said wafer.
 10. Thescanner element as claimed in claim 9 in which said separate memberssurround said aperture at opposite ends thereof and come in closeproximity thereto.
 11. The scanner element as claimed in claim 9 inwhich said separate members comprise electrodes adopted to be connectedto a Coulter particle device.
 12. In particle studying apparatus whichincludes a vessel having an aperture therein through which fluidscarrying suspensions of particles are adapted to pass, the passage of aparticle through the aperture resulting in a change of the impedance ofthe fluid in the aperture, and in which electrical means are provided todetect the passage of particles in terms of the change in impedance, theimprovement comprising, a scanner element which comprises a wall ofinsulating material having an orifice therein, a substantially inertwafer having a flat disc configuration and being of electricallyinsulative material and having thermal conductivity of at least 50Btu/hr/sq ft/*F/ft and having an aperture therein secured to said wallin face-to-face engagement and with the aperture and orifice aligned,wherein excessive noise in the aperture is reduced, the signal-to-noiseratio is improved, and the sensitivity of the scanner elmement isincreased.
 13. The structure as claimed in claim 12 in which theaperture is of right cylindrical configuration and having a sharpentrance edge.
 14. The structure as claimed in claim 12 in which thewafer is formed of an oxide of beryllium.
 15. The structure as claimedin claim 13 in which the wafer is formed of beryllium oxide.
 16. Anaperture tube for use with a Coulter particle apparatus comprising anelongate glass tube having an orifice in its wall adjacent the lower endthereof, an electrically insulative wafer being of a flat discconfiguration and having thermal conductivity of at least 50 Btu/hr/sqft/*F/ft engaged to said wall and blocking said orifice and there beinga through aperture in said wafer aligned with said orifice, whereinexcessive noise in the aperture is reduced, the signal-to-noise ratio isimproved, and the sensitivity of the aperture tube is increased.
 17. Theaperture tube as claimed in claim 15 in which the wafer is formed of anoxide of beryllium.